Conventionally, a permanent magnet type motor having a permanent magnet disposed at the rotor is employed in various fields, and used as a driving source for electric vehicles and hybrid vehicles.
For the driving source of such electric vehicles and hybrid vehicles, the vehicle running performance of low revolution-high power and high revolution-low power is required.
The torque produced by the motor generally depends upon the magnetic flux flowing to the stator from the rotor and the armature current flowing to the stator winding.
The magnetic flux flowing across the stator and rotor is determined by the employed magnet and the like. The magnetic flux is maintained constant independent of the rotational speed. The rotational speed is determined by the armature current. However, since the armature current is determined depending upon the voltage from the power source such as an inverter, the speed of revolution becomes highest when the voltage of the armature winding matches the maximum voltage of the power supply voltage.
When constant power driving is to be conducted based on a constant power supply voltage in such a permanent magnet type motor, various methods are proposed for the purpose of further increasing the aforementioned highest speed of revolution to improve the running performance as well as to increase the power at a low revolution speed, such as the so-called “field weakening control” and “field strengthening control” (Japanese Patent Laying-Open Nos. 2005-65385, 6-351206, 7-336980, 2002-78306, and 7-288960, as well as; “Some Considerations on Simple Non-Linear Magnetic Analysis-Based Optimum Design of Multi-pole Permanent Magnet Machines” by Yoshiaki Kano, Takashi Kosaka, and Nobuyuki Matsui in IEEJ Trans. IA, Vol. 123, No. 3, pp. 196-203 (2003) (hereinafter, referred to as Non-Patent Document 1); and “Some Investigations into Performance of Hybrid Motor with Novel Construction” by Jin Zheguo, Takashi Kosaka, and Nobuyuki Matsui in the Proceedings of National Conference of the IEE of Japan 2005 (hereinafter, referred to as Non-Patent Document 2).
For example, Non-Patent Documents 1 and 2 propose a hybrid motor based on a combination of a powder-molded magnetic element and a toroidal field coil for a multi-pole permanent magnet.
The hybrid motor disclosed in Non-Patent Documents 1 and 2 includes two rotor cores fixedly installed on the rotational shaft with a distance therebetween, a ring magnet fixedly installed on the rotational shaft located between the rotor cores, and a toroidal field coil.
On the surface of each rotor core are formed a plurality of rotor teeth protruding outwards, and a recess located between each of the rotor teeth.
In this hybrid motor, the magnetic flux for rotating the rotor by the flow of the magnetic flux from the ring magnet running between the rotor and stator, and the magnetic flux generated by the supply of current to the toroidal field coil contribute to the rotor rotation.
However, it has become difficult for the hybrid vehicles disclosed in Non-Patent Documents 1 and 2 to generate sufficient magnetic flux to cause the rotor to rotate at high speed of a level that allows usage as a driving force of the hybrid vehicle and electric car based on the magnetic flux from the toroidal field coil and ring magnet.